In a way. An extreme could be not loading content until the ad is loaded, and there is some type of check to verify that it did actually load (not blocked). I

In a way. An extreme could be not loading content until the ad is loaded, and there is some type of check to verify that it did actually load (not blocked). If someone is determined enough, they could simulate the ad being loaded (eg, maybe it actually IS even loaded, but the DOM element is hidden in one of dozen different ways which the ad-loaded-verification code doesn't check for). Essentially the problem boils down to running code (HTML, Javascript, CSS, and even Flash) in an untrusted environment (the user's browser). The site sends the user bits of data to be displayed, but there's ultimately no way to know if the user's browser ACTUALLY displays those in the browser window, let alone to the user's eyeballs. As a content provider, you can get very, very clever, but you can never get to 100%, and there will always be a motivated user to work on an adblocker that circumvents everything. It's really not any different from DRM (as someone else in thread pointed out). The movie/music/TV/games industry has spent billions of dollars coming up with various schemes of DRM, yet you can still download basically anything (stripped of its DRM, of course) within days of release, if not earlier.